If the ASR malfunction lamp is on but the lamp for ABS is not, then I believe LH Mode is caused by a malfunction in the Electronic Accelerator (EA). In this situation LH Mode can occur with the key on and the engine off, and you can tell that it has occurred by looking at the throttle linkage with the air cleaner housing removed.

The EA controller gets throttle position feedback from a potetiometer in the throttle actuator. If this signal fails to verify that the throttle position is following the command from the controller to the actuator, then the electronic control is disabled and the throttle goes to LH Mode.

If the EA is your problem, then you can restore normal power with a simple modification to the throttle linkage. Refer to the attached diagram.

The rod #97 rotates to move the plate in the throttle body (picture). Normally #97 moves by the throttle actuator and rod #109. But in LH Mode the actuator is dead, and #97 moves through an arm that engages it at its back end (circled). With the air cleaner assembly removed and the throttle closed, in LH mode you can see that as the gas pedal is pressed the arm has to move through a gap before it begins to rotate #97. You can eliminate this slop by wedging a rubber hose or something else into the gap. Another option which I prefer is to change the length of one of the rods in the linkage (I think it is #88) to reposition the arm and eliminate the gap.

ASR can be disabled by interrupting the engine-running signal input to the ASR control module. This is how Renntech does it and is also how it is done in most if not all of the threads in the forums.

On a car with CIS fuel injection the engine-running signal is the blue/white wire to terminal #15 of the ASR module electrical connector. If ASR is disabled in this manner, then the malfunction lamps for ABS and ASR in the instrument cluster are on all the time. To disable those, you interrupt wires to terminals #17 and #29. If no switch is being used to disable the ASR and it is instead off all the time, then you would also probably want to disable the "!" indicator in the speedometer. To do that you interrupt wire #15.

Yes, defeating the ASR will not make your LH mode go away, you could just un-plug a speed sensor to defeat btw....take a look at your engine wiring; both upper and lower harnesses. If you peel apart some of the wrapped wires, the insulation may be all gone and is time to remove that vital part of the system from the diagnosis with a new set. Not exactly cheap...but the new wires are much better made now as opposed to OE from 1991. A very common problem on this vintage MB motor too.

Your LH problem may even be as simple as the $25 brake light switch under the pedal also. Having the code(s) pulled MAY find where the problem is, but it also may not. BTDT on M119s.

Could def. also be the EA... big bucks; $1100+ ...but I think Beckmann is re-building them now for much cheaper than new ones if it is that unit. Good luck...hope it's simple !